1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to improvement of dimples of golf balls.
2. Description of the Related Art
Golf balls have a large number of dimples on the surfaces thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. This phenomenon is referred to as “turbulization”. Due to the turbulization, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulization promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. Excellent dimples efficiently disturb the air flow. The excellent dimples produce a long flight distance.
The ratio of the sum of the areas of dimples to the surface area of a phantom sphere of a golf ball is referred to as an occupation ratio. In general, in a golf ball having a high occupation ratio, the degree of turbulization is great. A golf ball having a high occupation ratio has excellent flight performance.
It is known that the degree of turbulization is great in a golf ball in which the diameters of dimples are less variable. The golf ball has excellent flight performance.
In order to increase an occupation ratio, it is necessary to locate a small-diameter dimple in a narrow zone surrounded by a plurality of dimples. The presence of the small-diameter dimple causes an increase in variation of the diameters of dimples. Increasing an occupation ratio and suppressing the variation of the diameters are incompatible with each other.
The degree of turbulization also depends on the cross-sectional shapes of dimples. In a golf ball in which dimples are too deep, turbulization is insufficient. Also in a golf ball in which dimples are too shallow, turbulization is insufficient.
There have been various proposals for the cross-sectional shapes of dimples. JPS62-192181 (U.S. Pat. No. 4,813,677) discloses a golf ball that has dimples having large diameters and large depths and dimples having small diameters and small depths.
JPH2-134175 (U.S. Pat. No. 5,033,750) discloses a golf ball in which the difference between a value obtained by dividing the diameter of a dimple by the depth thereof and a value obtained by dividing the diameter of another dimple by the depth thereof is equal to or less than 0.3.
JPH3-198875 (U.S. Pat. No. 4,979,747) discloses a golf ball that has dimples having large diameters and small depths and dimples having small diameters and large depths.
JPH4-231079 (U.S. Pat. No. 5,016,887) discloses a golf ball in which values obtained by dividing the depths of all dimples by the diameters thereof are the same.
JPH5-237202 (U.S. Pat. No. 5,158,300) discloses a golf ball in which the edge angles of all dimples are the same.
The greatest interest to golf players concerning golf balls is flight distance. In light of flight performance, there is room for improvement in the shapes of dimples. An object of the present invention is to provide a golf ball having excellent flight performance.